Light sources which produce a high color rendering index (CRI) have many applications in the lighting industry. CRI is an indication of the vibrance of the color of light being produced by a light source, and is an indication of the amount of color shift exhibited by the light source upon reflectance. The higher the CRI value for a particular light source, the more white the light and the better or more true that the light source renders the colors of various objects it is used to illuminate. One method for determining CRI is referred to as “R8” and involves the use of eight color chips or palettes which are illuminated by the light source. A CRI value is determined for each of the eight color chips being illuminated, with a CRI of 100 indicating perfect color rendering for that particular color chip as compared to an ideal planckian radiator or daylight emission spectrum. The eight CRI values are then averaged to produce a single CRI value for the light source being evaluated. Alternatively, other CRI evaluation techniques, such as the use of 14 color chips, may be used. High CRI light, or high color rendering light accents the various colors in objects which are to be illuminated.
Another factor to be considered in evaluating a light source is the concept of “color temperature,” which is an indication of how much color is perceived to be produced by the light source. Higher color temperature values are used to refer to “cool” color temperatures, where the color of light produced is perceived to have a blue-ish tint. The peak wavelength of the spectral power distribution of the light is inversely proportional to the color temperature of the light. Lower color temperature values are used to refer to “warm” white light which is perceived as having a more yellow color.
Traditional solid state LED lights used to create white light sources have typically used blue LEDs having an excitation near 440-460 nm in conjunction with 573 nm yellow phosphors overlaid on the LEDs. It should be noted that the designation of the phosphor refers to the color (expressed in wavelengths) of the light emitted by the phosphor based on the light incident to the phosphor. Thus, “yellow” phosphor refers to phosphor which emits light in the yellow range, approximately 573 nm. The phosphor layer converts lower wavelength light, such as that generated by the LEDs, into higher wavelength light. This is commonly referred to as luminescence, or phosphor luminescence.
However, the resulting light tends to be a “cool” light, having a color temperature in the range of 4000-4700 Kelvin (K). Furthermore, the CRI of such lights tends to be relatively low, often in the range of CRI value of 60.
Thus, there is a need to produce “warm” white and “cool” white light using LED light sources, which also exhibits a relatively higher CRI value. There is also a need to generate the light efficiently, as measured by the Light Efficiency Ratio (“LER”), also referred to as the Light Efficacy, with higher efficiency being preferable.